CANBERRA: Australia, the world's biggest iron ore producer, revised down its forecast for export revenue for the steelmaking ingredient on Tuesday, a fresh sign the mining sector is losing steam as a slowdown in top customer China drives down iron ore prices. The Bureau of Resources and Energy Economics (BREE) forecast iron ore exports in the year to June 30, 2013, would be 509 million tonnes, little changed on its June forecast of 510 million tonnes and up 8 percent on the previous year.